One type of optical disk system employs an "access system" for positioning the optical head radially, providing a very coarse tracking control. A fine tracking system, including an optical tracking servo and a tracking position servo, moves only the objective lens of the optical head. The objective lens is mounted in an electro-magnetic actuator on light springs, permitting two axis of freedom, one vertical for focusing, one radial for tracking. The usable range of tracks which can be reached by moving only the lens is referred to as a "tracking window".
The two tracking servos play the following roles during drive operation:
1. Optical Tracking Servo--This servo uses an error signal developed optically by the head, indicating lens position relative to a track on the disk, to maintain the lens position centered over the track.
2. Tracking Position Servo--This servo system is used primarily during access system moves. The natural response of the actuator is highly under-damped. Therefore, when the entire head is moved abruptly from one position to another, the actuator tends to ring at its natural frequency for an excessive period of time. Among other problems, the resulting large cross-track velocity at the center of the lens travel prevents the optical tracking servo from acquiring lock properly.
The position servo uses an error signal, developed from lens position detectors in the actuator, to hold the lens centered in the actuator. When the drive control electronics is about to move the head, the position servo is activated to hold the lens centered during the access system move. When the access system move is complete, the position servo is turned back off, and the optical tracking servo is locked.
The tracking system also includes a sub-system designed to reduce the effects of disk to spindle decentering. This "eccentricity" sub-system measures the magnitude and phase of the disk decentering when a disk is first clamped, and generates a matched, compensating sine wave thereafter. The optical tracking servo utilizes the sine wave as a feed forward, to reduce the residual detracking due to disk decentering. The position servo uses the sine wave as a "commanded position", so that when the drive switches from the tracking position servo back to the optical tracking servo, the lens will already be in the correct position with the correct velocity, on the eccentricity sine wave. This avoids a step change in the actuator drive when the servo mode is switched from position servo to optical tracking servo. Thus, there are two paths for the eccentricity compensating sine wave to reach the actuator.
The purpose of the present invention is to automatically adjust the gain of one of these two paths, so that the actuator sensitivity to the eccentricity compensating sine wave is the same for both paths.